Their reasoning is probably security. They're working under the assumption your app takes untrusted input in some way, maybe over the network. Which isn't a bad assumption, I mean almost all apps do. Very few apps are true self-contained applications, like a calculator.
So then if there happens to be some vulnerabilities in an older Android SDK then your app is susceptible. They could patch back security but that's expensive after a while. Easier to force app makers to update their apps.
So then if there happens to be some vulnerabilities in an older Android SDK then your app is susceptible. They could patch back security but that's expensive after a while. Easier to force app makers to update their apps.